The present invention relates to a monitoring apparatus for an optical fiber amplifier for amplifying an optical signal in a communication system using an optical fiber transmission path.
In a communication system using an optical fiber transmission path, as an optical amplifier for directly amplifying a transmitting optical signal, an optical fiber amplifier having an arrangement shown in FIG. 3 is known. In FIG. 3, reference numerals 1 and 11 denote optical connectors; 2, an optical multiplexer for multiplexing excited light from an excitation light source 3 with light incident via the optical connector 1; 4 and 6, optical isolators; 5, an erbium-doped optical fiber for amplifying the light incident via the optical isolator 4; 7, an optical passive component connected to the output side of the erbium-doped optical fiber 5 via the optical isolator 6 to branch reflected light; 9, a light-receiving element for receiving the light branched by the optical passive component 7; and 12, an optical fiber transmission path on the output side connected to the optical connector 11.
In the conventional optical fiber amplifier having this arrangement, an optical signal incident from an optical fiber transmission path (not shown) on the input side via the optical connector 1 is multiplexed by the optical multiplexer 2 with excited light output from the excitation light source 3, and then incident on the erbium-doped optical fiber 5 via the optical isolator 4. The optical signal amplified by the erbium-doped optical fiber 5 emerges to the optical fiber transmission path 12 via the optical isolator 6, the optical passive component 7, and the optical connector 11. At this time, reflected light from the output side is branched by the optical passive component 7 to be incident on the light-receiving element 9. The optical passive component 7 and the light-receiving element 9 constitute a reflection monitoring circuit 20 for monitoring the reflected light amount from the output side to detect an abnormality on the transmission path.
In this arrangement, if the optical connector 11 at the output portion is disconnected from the erbium-doped optical fiber 5, part of light output from the optical fiber amplifier is Fresnel-reflected by the end face of the optical connector 11. Then, the reflected light is branched by the passive component 7 to be incident on the light-receiving element 9. When the optical connector 11 is kept connected to the optical fiber transmission path 12, the reflected light amount of light output from the optical fiber amplifier on the end face of the connector is small, so that the power of the reflected light incident on the light-receiving element 9 is small. When the optical connector 11 is disconnected from the optical fiber transmission path 12, the reflected light amount of light output from the optical fiber amplifier on the end face of the optical connector 11 becomes large to increase the power of the reflected light incident on the light-receiving element 9. When the power of the reflected light incident on the light-receiving element 9 reaches a predetermined level or higher, the reflection monitoring circuit 20 detects the disconnection of the optical connector 11. Also when the transmission path after the optical passive component 7 fractures, the power of the reflected light incident on the light-receiving element 9 increases, and the reflection monitoring circuit 20 detects the fracture of the transmission path.
In the conventional reflection monitoring circuit 20, when light having power exceeding a certain threshold is incident on the optical fiber transmission path 12, stimulated Brillouin scattering (SBS) occurs in the optical fiber transmission path 12, and the back-scattering light enters the light-receiving element 9 via the passive component 7. As a result, the power of the light incident on the light-receiving element 9 increases. Even if the optical connector 11 is normally connected to the optical fiber transmission path 12, the reflection monitoring circuit 20 erroneously detects that the optical connector 11 is disconnected.